A young girl, suffering from diabetes type 1, sets the bolus of her insulin pump which monitors the consumption of carbohydrates in her body in Berlin, Germany on Feb. 19. Jens Kalaene / Picture-alliance/dpa via AP, file

Dec. 15, 2015 / 2:21 PM GMT / Updated Dec. 15, 2015 / 2:21 PM GMT

By Janine Wolf

For the first time in more than 20 years, researchers are witnessing a steady drop in new diabetes diagnoses among the American adult population—down by about a fifth from 2009 to 2014—according to new data released by the Center for Disease Control and Prevention (CDC).

"We used to think that one size fits all, but now we know that that's not true."

But in the breakdown of data from the CDC, which reveals a clear decline of new cases among white, black, and Hispanic adults, the report did not offer any insight about the current stance of Asian Americans—a group that the National Institutes of Health (NIH) and CDC previously reported has the "highest proportion of diabetes that was undiagnosed among all ethnic and racial subgroups."

“Among everyone with diabetes in the U.S., about one in three were unaware that they had the condition,” Dr. Andy Menke, the NIH study’s first author, told NBC News. “This was even higher among Asians, where one in two were unaware that they had the condition.”

Though the new CDC data reports a decline in new diagnoses, the prevalence of diabetes across all American adults is more than double what it was 20 years ago.

Dr. Edward Gregg, chief of the epidemiology and statistics branch of the CDC's Division of Diabetes Translation, tells NBC News that the reduction of new cases of diabetes in the United States is "encouraging," but said that it is more difficult to gauge where the Asian-American population stands at the moment.

"The next steps are to increase the amount of clinical research and data on this diverse population, while simultaneously pushing for policy change that will positively impact health outcomes."

“In terms of the incidence rate in Asian Americans, we found that the shape of the curve actually looks similar," Gregg said. "We wouldn’t say that it’s a statistically significant decrease, but it does appear that those rates have leveled off in Asian Americans, as well.”

“Unfortunately, these studies did not have adequate statistical power to accurately estimate cases of diabetes among Asian Americans in previous decades, so it's not possible to look at long-term trends in this population," Menke said.

While increased body weight indicates a risk for type 2 diabetes, the most common form of diabetes, overall weight gain is not a typical symptom for many Asian Americans.

In the U.S., scientists use body mass index (BMI), which compares an individual’s height to his or her overall weight, as one possible indicator of diabetes. A BMI of 25 to 30 is considered overweight, and a BMI of 30 or greater is deemed obese.

But according to a new study published last year by the American Diabetes Association (ADA), Asian Americans should be tested for diabetes with BMI levels as low as 23—which is lower than that of the general U.S. population.

"We've never differentiated based on ethnicity because frankly, in the past, we haven't done a good job evaluating different ethnicities," Dr. Jane Chiang, senior vice president of medical and community affairs at the ADA, told NPR last year. "We used to think that one size fits all, but now we know that that's not true."

Due to variations in body composition between Asian and white adults, results of relevant studies may be inapplicable to the Asian population.

"The next steps are to increase the amount of clinical research and data on this diverse population, while simultaneously pushing for policy change that will positively impact health outcomes,” Ho Luong Tran, president of the National Council of Asian Pacific Islander Physicians and lead coordinator of the AANHPI Diabetes Coalition, said in a statement last year.

Since addressing the issue of BMI variations between Asian Americans and the general population in the ADA position statement, the ADA has made alterations to the BMI cut-off point for type 2 diabetes screening.

“What this does is to help us, as a society, identify those who are at risk for type 2 diabetes who might otherwise not have been identified because of their lack of appearance of obesity," Dr. William C. Hsu, lead author of the ADA study, said in last year's statement.